High performance circuits, especially those used for radio frequency chips, favor the use of heterojunction bipolar transistors (HBTs) to provide high maximum oscillation frequency fMAX and high transient frequency fT, also referred to as “cutoff frequency”. HBTs have a structure in which the base of the transistor includes a relatively thin layer of single-crystal semiconductor alloy material. As an example, an HBT fabricated on a substrate of single-crystal silicon can have a single-crystal base formed of silicon germanium (SiGe) having substantial germanium content and profile to improve high-speed performance. Such HBT is commonly referred to as a SiGe HBT.
The juxtaposition of alloy semiconductor materials within a single semiconductor crystal is called a “heterojunction.” The heterojunction results in an increase in current gain. This increase in gain allows a significant increase in base doping, and corresponding decrease in base resistance, which would otherwise result in a decrease in current gain. Further, by varying the composition of the semiconductor alloy in the base as a function of position, a significant quasi-static field may be created that enhances the velocity of charge carriers in the base. Increased velocity, in turn, enables higher gain and cutoff frequency to be achieved than in transistors having a uniform semiconductor alloy composition throughout.
To increase the performance of an HBT, it is desirable to increase both the transient frequency fT and the maximum oscillation frequency fMAX. FMAX is a function of fT and of parasitic resistances and parasitic capacitances (collectively referred to herein as “parasitics”) between elements of the transistor according to the formula fMAX=(fT/8π*Ccb*Rb)1/2.
The parasitics of the HBT include the following parasitic capacitances and resistances, as listed in Table 1:
TABLE 1Ccbcollector-base capacitanceCebemitter-base capacitanceRccollector resistanceReemitter resistanceRbbase resistance
The most significant parasitics are the collector-base capacitance Ccb and the base resistance Rb because they provide an electrical feedback path between the output and input of the transistor, reducing power gain and thus gain-dependent figures of merit including fMAX. Their values are typically larger than the other parasitics, making their effects on fT and fMAX more pronounced. Thus, it is desirable to provide an HBT structure and method by which Ccb and Rb are significantly reduced.
An example of a state of the art heterojunction bipolar transistor (HBT) structure containing parasitics is illustrated in FIG. 1. As depicted in the cross-sectional view therein, an ideal or “intrinsic” device consists of a one-dimensional slice downward through the centerline 2 of the HBT, through emitter 4, intrinsic base layer 3, and collector 6. The emitter 4 is generally heavily doped with a particular dopant type, (e.g. n-type), and generally consists essentially of polycrystalline silicon (hereinafter, “polysilicon”). The intrinsic base 3 is predominantly doped with the opposite type dopant (e.g. p-type) than the emitter 4, and generally less heavily. The collector 6 is doped predominantly with the same dopant (e.g. n-type) as the emitter 4, but generally less heavily than the intrinsic base 3. Region 5 represents the depletion region disposed between the intrinsic base 3 and the collector 6, due to the p-n junction between the base and collector, which have different predominant dopant types. Region 7 represents the depletion region disposed between the intrinsic base 3 and the emitter 4, due to the p-n junction between the base and emitter, which have different predominant dopant types. Often, the intrinsic base 3 is formed of silicon germanium (SiGe), which is epitaxially grown on the surface of the underlying collector 6.
The ideal structure itself contains two capacitances that impact performance. There is the intrinsic emitter-base capacitance CBE,I at the junction 7 between the emitter 4 and the base 3. In addition, there is an intrinsic collector base capacitance CCB,I at the junction 5 between the collector and the base. These capacitances are related to the areas of the respective junctions, as well as to the quantities of dopant on either side of the respective junctions. Although these capacitances impact the power gain of the transistor, they are an inextricable part of the ideal transistor structure and thus cannot be fully eliminated.
Since a one-dimensional transistor, free of all material beyond the intrinsic device, cannot be realized in a practical process, typically a transistor contains additional parasitics stemming from interaction between the intrinsic device and other material structures in which the intrinsic device is embedded. Such structures help to provide electrical access and heat transfer to and from the intrinsic device. Among such additional parasitics, which have a key impact upon power gain is the extrinsic collector base capacitance CCB,E of which CCX and CRX are components, as shown in FIG. 1. The first component capacitance CCX results from interaction between the extrinsic base of the device and the collector pedestal. The second component capacitance CRX results from interaction between the extrinsic base of the device and the bulk substrate portion of the collector, between the edge of a shallow trench isolation 9 and the collector pedestal 6. An additional component capacitance CPB is the capacitance of the extrinsic base and substrate where separated by the STI. Provided that a given HBT fabrication process results in an STI having sufficient thickness to avoid substantial CPB, the parasitic capacitances CCB,I, CCX and CRX contribute more significantly to the overall collector base capacitance Ccb.
As illustrated in FIG. 2, the extrinsic base resistance Rb is a second important parasitic element, representing the series resistance between the external base contact and the intrinsic base film. The components of the base resistance Rb include: Rint, which is a function of the size of the emitter and the intrinsic base profile; Rsp+link, which is a function of the width of the spacer separating the raised extrinsic base from the emitter, as well as the interface quality of the link between the intrinsic base and the raised extrinsic base; Rpoly, which is function of the thickness, doping and alignment of the edge of the silicide (when present) to the polysilicon layer 8 of the raised extrinsic base; and Rsilicide, which is a function of the dimension of the polysilicon 8 over which a silicide 11 is disposed. The parasitic resistances Rpoly and Rsilicide contribute significantly to overall base resistance Rb.
Typically, moving the extrinsic base elements closer to the intrinsic device reduces Rb. However, such an approach tends to increase the extrinsic collector base capacitance CCB,E, creating a fundamental tradeoff between the two parasitics and making it hard to improve overall power gain. Narrowing the collector pedestal itself can also reduce CCB,E. Such a reduction is difficult to achieve, however, since the pedestal is typically formed by implantation of dopants, which tend to scatter laterally during implantation and to diffuse laterally during the typical heating that a transistor experiences during fabrication. Narrowing the collector pedestal also increases the series resistance (RC) of the collector pedestal, impacting high frequency performance. Thus, it is desirable to avoid narrowing the collector pedestal, which adversely impacts RC.
A structure and method of confining the lateral dimension of the collector pedestal near the point of interaction with the extrinsic base, while maintaining low RC and preserving tolerance against process thermal cycle would be of major advantage in improving the high-frequency gain of a bipolar transistor.
Therefore, it would be desirable to provide a structure and method of fabricating a bipolar transistor having reduced extrinsic collector base capacitance CCB,E without significantly impacting the extrinsic emitter base resistance Rb or the collector resistance RC, so as to achieve superior high-frequency power gain.
Commonly assigned, co-pending U.S. patent application Ser. No. 10/249,299 describes an HBT having reduced collector-base capacitance and resistance by vertically interposing first and second shallow trench isolation (STI) structures between the collector which underlies the STI and the raised extrinsic base which overlies the STI.
It would further be desirable to increase the transient frequency fT and maximum oscillation frequency fMAX through change in one or more of the vertical profiles of the collector, base, emitter and/or the junctions between them.